PDC cutter for high compressive strength and highly abrasive formations

ABSTRACT

A PDC cutter utilizes the combination of an elliptical shape with higher thermal resistance obtained through leaching to provide a cutter which is more effective than a cutter using either concept alone. The PDC cutter includes a tungsten carbide portion with protrusions extending from a surface thereof in a pattern. The diamond volume is mounted to the surface wherein the protrusions allow for the diamond volume to be larger about a perimeter edge of the cutter and smaller/shallower in a center region of the cutter. The protrusions providing surfaces for the diamond layer (table) to bond to the underlying tungsten carbide portion. With this configuration, diamond volume is maximized around the edge of the cutter.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. ProvisionalApplication for Patent No. 60/751,835 filed Dec. 20, 2005, thedisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a PDC cutter used for drillingboreholes into the earth.

2. Description of Related Art

Drilling high compressive strength rocks that are highly abrasive hasalways been a challenge for PDC cutters. When a rock has a highcompressive strength, high force is required for the cutter to penetratethe rock. High cutting forces cause problems. When the high cuttingforces are transmitted to the drill string, drilling problems occur. Forexample, the drill string can buckle or bow causing unwanted deviationof the well. High forces cause problems within the cutter. The higherforces cause more frictional heating. The heating causes rapid wear ofthe PDC cutter. Heating of the cutter causes thermal stresses within thecutter and subsequent cracking. These problems make drilling theformation with PDC cutters uneconomical.

Currently high compressive strength/highly abrasive formations can bedrilled with impregnated drill bits. Impregnated drill bits can resistthe higher heat of high bit weights. However, they take small depths ofcut. To drill sufficiently fast, they must be turned at a high rpm.Turning at a high rpm requires special equipment, increasing the cost ofdrilling operations.

Currently high compressive strength/highly abrasive formations can bedrilled with insert rock bits. These bits require high levels of weightto drill efficiently. The high WOB increases the risk of the drillingproblems mentioned above. It also causes rapid wear of the seals andbearings of the rock bit. This shortens bit life requiring the operatorto frequently change the bit.

The following references are related to leached cutters: 6,410,085 25Jun. 2002 Griffin et al 6,435,058 20 Aug. 2002 Matthias et al 6,481,51119 Nov. 2002 Matthias et al 6,544,308 8 Apr. 2003 Griffin et al6,562,462 13 May 2003 Griffin et al 6,585,064 1 Jul. 2003 Griffin et al6,589,640 8 Jul. 2003 Griffin et al 6,592,985 15 Jul. 2003 Griffin et al6,601,662 5 Aug. 2003 Matthias et al 6,739,214 25 May 2004 Griffin et al6,749,033 15 Jun. 2004 Griffin et al 6,797,326 28 Sep. 2004 Griffin etal 6,878,447 12 Apr. 2005 Griffin et al 6,861,098 1 Mar. 2005 Griffin etal 6,861,137 1 Mar. 2005 Griffin et al

The following reference is related to an elliptical cutter: 6,808,031 26Oct. 2004 Wilmot et al

The foregoing references are incorporated herein by reference.

PDC cutters are formed from a mix of materials processed underhigh-temperature and high pressure into a polycrystalline matrix ofinter-bonded super hard carbon based crystals. A common trait of PDCcutters is the use of catalyzing materials during their formation. Theresidue from the catalyzing materials imposes a limit upon the maximumuseful operating temperature of the element while in service.

The common form of PDC cutter is a two or more layer PDC cutter where alayer of polycrystalline diamond is integrally bonded to a substrate oftungsten carbide. The PDC cutter may be brazed to a carrier, often alsoof cemented tungsten carbide. This is a common configuration for PDC'sused as cutting elements in drill bits.

PDC cutters are most often formed by sintering diamond powder with acatalyst in a high-pressure, high-temperature press. One particularmethod is disclosed in U.S. Pat. No. 3,141,746, the disclosure of whichis hereby incorporated by reference. In the typical process formanufacturing PDC cutters, diamond powder is loaded in an assembly witha preformed tungsten carbide substrate incorporating cobalt. Theassembly is then subjected to very high temperature and pressure in apress. During this process, cobalt migrates from the substrate into thediamond layer and acts as a catalyst, causing the diamond particles tobond to one another with diamond-to-diamond bonding, and also causingthe diamond layer to bond to the substrate.

The completed PDC cutter has a matrix of diamond crystals bonded to eachother with many interstices containing the catalyst metal. The catalystmetal is most commonly cobalt. The diamond crystals form a continuousmatrix of diamond, and the interstices form a continuous matrix ofcatalyst metal. These two matrices are usually known as the diamondlayer or diamond table. The diamond table is in turn bonded to thetungsten carbide substrate.

Typical PDC cutters have a diamond table that is 85% to 95% diamond byvolume. The other 5% to 15% volume is metal catalyst. The PDC cuttersare subject to thermal degradation due to differential thermal expansionbetween the metal catalyst and the diamond matrix. This type of thermaldegradation begins at temperatures of about 400 degrees C. Uponsufficient expansion the diamond-to-diamond bonding may be ruptured andcracks and chips may occur.

Also in polycrystalline diamond, the presence of the metal catalyst inthe interstices leads to another form of thermal degradation. Due to thepresence of the metal catalyst, the diamond is caused to graphitize astemperature increases, typically limiting the operation temperature toabout 750 degrees C.

To reduce thermal degradation, “thermally stable” polycrystallinediamond components have been produced. A typical configuration isdescribed in U.S. Pat. No. 4,224,380, the disclosure of which is herebyincorporated by reference. In this type of thermally stable PDC cutterthe cobalt is leached from the interstices. While this increases thetemperature resistance of the diamond to about 1200 degrees C, theleaching process also removes the cemented carbide substrate. Becausethere is no integral substrate or other bondable surface, there aresevere difficulties in mounting such material for use in operation.

The fabrication methods for this “thermally stable” PDC cutter typicallyproduce relatively low diamond densities, of the order of 80% or less.This low diamond density enables a thorough leaching process, but theresulting finished part is typically relatively weak in impact strength.

In an alternative form of thermally stable polycrystalline diamond,silicon is used as the catalyzing material. The process for makingpolycrystalline diamond with a silicon catalyzing material is quitesimilar to that described above, except that at synthesis temperaturesand pressures, most of the silicon is reacted to form silicon carbide,which is not an effective catalyzing material. The thermal resistance issomewhat improved, but thermal degradation still occurs due to someresidual silicon remaining. Again, there are mounting problems with thistype of PDC cutter because there is no bondable surface.

Efforts to combine thermally stable PDC's with mounting systems to puttheir improved temperature stability to use have not been as successfulas hoped due to their low impact strength. For example, various ways ofmounting multiple PDC cutters are shown in U.S. Pat. Nos. 4,726,718;5,199,832; 5,025,684; 5,238,074; 6,009,963 herein incorporated byreference for all they disclose. Although many of these designs have hadcommercial success, the designs have not been particularly successful incombining high wear and/or abrasion resistance while maintaining thelevel of toughness attainable in non-thermally stable PDC.

Other types of diamond or diamond like coatings for surfaces aredisclosed in U.S. Pat. Nos. 4,976,324; 5,213,248; 5,337,844; 5,379,853;5,496,638; 5,523,121; 5,624,068 all herein incorporated by reference forall they disclose. Similar coatings are also disclosed in GB PatentPublication No. 2,268,768, PCT Publication No. 96/34,131, and EPCPublications 500,253; 787,820; 860,515 (all herein incorporated byreference for all they disclose) for highly loaded tool surfaces. Inthese publications, diamond and/or diamond like coatings are shownapplied on surfaces for wear and/or erosion resistance.

Some attempts have been made to improve the toughness and wearresistance of these diamond or diamond like coatings by application to atungsten carbide substrate and subsequently processing in ahigh-pressure, high-temperature environment as described in U.S. Pat.Nos. 5,264,283; 5,496,638; 5,624,068 herein incorporated by referencefor all they contain. Although this type of processing may improve thewear resistance of the diamond layer, the abrupt transition between thehigh-density diamond layer and the substrate make the diamond layersusceptible to wholesale fracture at the interface at very low strains.This translates to very poor toughness and impact resistance in service.

The most successful solution to the above problems is a conventional PDCcutter with only a portion of the catalyst metal removed. This type ofcutter has greatly improved resistance to thermal degradation withoutloss of impact strength. In more detail, a conventional PDC cutter has atable with a continuous diamond matrix with metal catalyst remaining inthe interstices. The working surfaces have had the cobalt leached to adepth of a few thousandths of an inch.

With this configuration, the PDC cutter retains the strength of aconventional PDC cutter. The PDC cutter can also easily be attached tothe drill bit on the carbide side of the cutter. However, the workingsurfaces have a layer of the diamond that is more resistant to thermaldegradation. This gives a good combination of properties that exceed theperformance properties of conventional PDC cutters.

PDC cutters with a thermally stable layer have exceeded the performanceof conventional PDC cutters. They have expanded the application of PDCbits into harder and more abrasive formations. However, they have notbeen successful in high compressive strength highly abrasiveapplications. In these applications, the PDC cutters continue to wear ata high rate, rendering the drill bit uneconomical for use.

A PDC cutter with improved durability uses elliptical shaped cutters.These cutters have been marketed as “Oval” cutters. They are PDC cuttersthat have an elliptical form with a long (major) axis and a short(minor) axis. The elliptical cutters are shown diagrammatically in FIG.1 beside their conventional round cutter equivalents as a comparison.

Conventional cylindrical cutters are placed with the diamond tablefacing the direction of bit rotation. The round edge of the cutter ispushed into the formation by the weight on bit. When elliptical cuttersare used, the small end of the cutter is presented to the formation.This has the effect of a “sharper” edge. The sharper tip profile of theelliptical cutter form generates high point loading at lower weights onbit, increasing penetrating ability thereby enhancing ROP. Simply put,the elliptical cutter converts applied weight on bit into ROP moreefficiently than a round cutter. That is, less weight on bit is requiredto push the cutter into the formation than a bit made with conventionalcylindrical cutters.

The large surface area to point of contact ratio of the ellipticalcutter prolongs cutter life through improved thermal management. Thecutting tip is better preserved, and performance is maintained overlonger bit runs. As the elliptical cutter wears, it stays sharper than around cutter due to the development of a smaller carbide bearing areathan an equivalent round cutter. The smaller contact area maintainspoint loading and reduces heat build up in the substrate and diamondtable.

The success of this approach has been documented in field results. Byreplacing 13 mm round cutters on a bit with 19 mm×13 mm ellipticalcutters, the diamond volume (density) and cutter exposure (height), isincreased significantly. The use of the elliptical cutter form deliversincreased diamond volume without affecting the number of cutters on thebit, thus maintaining the cutter setting and ROP performance.

The use of increasingly thick diamond tables on elliptical cutters wasresisted until a design was perfected that capitalized on theadvantages, but overcame the downsides associated with thick diamond.

Thick diamond layers, offered by all cutter manufacturers, promised muchin terms of improved durability and increased run length. The realitywas that it proved difficult to retain the diamond layer on thesubstrate due to residual stresses inherent in the manufacturingprocess. The stresses often caused significant spalling, and in somecases, the total de-lamination of the diamond layer, resulting in earlybit failure.

In cases where the bond did not fail and the diamond table was retained,many PDC bits were being pulled for low penetration rates without heavywear. This was because normal progressive cutter wear resulted in alarge diamond bearing area that due to its size was unable to shear theformation. Effectively, as the cutter wore, the surface area of thediamond in contact with the formation became so enlarged that it couldno longer penetrate the formation. Successful shearing of formation isonly possible when the cutter can penetrate the rock. A sharp diamond“lip” is essential to enable this to happen.

SUMMARY OF THE INVENTION

While the concepts of leaching cutters and using elliptical shapedcutters have improved the performance of PDC bits, neither has beensuccessful in drilling high compressive strength formations that arehighly abrasive. The present invention utilizes the combination ofelliptical shape and higher thermal resistance through leaching toprovide a cutter which is more effective than a cutter using eitherconcept alone. There appears to be a synergy between the two concepts.

In an embodiment, a PDC cutter utilizes the combination of an ellipticalshape with higher thermal resistance obtained through leaching. Atungsten carbide portion includes protrusions which extend from asurface thereof in a pattern. The diamond volume is mounted to thesurface wherein the protrusions allow for better attachment as well asfor the diamond volume to be larger about a perimeter edge of the cutterand smaller/shallower in a center region of the cutter.

In an embodiment, a PDC cutter comprises: a tungsten carbide substratehaving a top surface including a protrusion pattern formed in a centerportion of the top surface; and a diamond table mounted to the topsurface of the tungsten carbide substrate. The diamond table is thickerat a perimeter of the top surface and thinner at the center portion ofthe top surface, and the diamond table is leached.

In another embodiment, a PDC cutter comprises: a tungsten carbidesubstrate; and a diamond table mounted to the top surface of thetungsten carbide substrate, the diamond table presenting a chiselforward shape. The cutter has an elliptical shape, and the diamond tableis leached.

In an embodiment, a PDC cutter comprises: a tungsten carbide substratehaving a top surface including a protrusion pattern; and a diamond tablemounted to the top surface of the tungsten carbide substrate. Thediamond table is thicker at a perimeter of the top surface and thinnerat the center portion of the top surface, and the diamond table isleached.

The invented configuration of PDC cutter seeks to increase drillingefficiency by the following: The shape of the PDC cutter requires lessweight on bit than conventional PDC cutters, lessening wear and crackingdue to frictional heating. It is composed of preferred PDC materials toincrease the cutters resistance to heat. The shape of the cutter isoptimized for high strength to reduce damage from high down hole forces.The shape of the cutter is optimized for easier cooling from the flow ofthe surrounding fluid. These combined factors enable the economicaldrilling of formations that were not previously drillable by PDCcutters. The PDC cutters increase the rate of penetration beyondimpregnated or rock bits.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be acquired by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIG. 1 diagrammatically shows elliptical cutters beside theirconventional round cutter equivalents as a comparison;

FIG. 2 is a perspective cut-away view of an elliptical PDC cutter inaccordance with an embodiment of the invention;

FIGS. 3A-3C show various views, side, top and perspective, respectively,of a plow shaped cutter in accordance with an embodiment of theinvention;

FIGS. 4A-4D which show various views, two side, top and perspective,respectively, of an elliptical PDC cutter with concave surface inaccordance with an embodiment of the invention;

FIGS. 5A-4C which show various views, side, top and perspective,respectively, of an elliptical PDC cutter with concave surface inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally speaking, the present invention relates to the use of aleached PDC cutter which has an elliptical cutter geometry. This cutterhas a unique diamond layer geometry and configuration that minimizes therisk of de-lamination, and delays the onset of wear, but as wearprogresses, allows the cutter to “lip” thus preserving the cutter's rockshearing capability.

As shown in FIG. 2, diamond volume is maximized around the edge of thecutter. Protrusions from the tungsten carbide portion 10 of the cutterform a pattern 12 (in this exemplary illustration, cross-hatched) whichallows for the diamond 14 volume to be larger about the perimeter edge,and smaller/shallower in the center region, while providing surfaces forthe diamond layer (table) to bond to the underlying tungsten carbidesubstrate. Again, the cutter has an elliptical shape and the diamondtable has been subjected to leaching. When the cutter is unworn, thisthicker diamond resists wear at the cutting tip as it is better able tocope with abrasion. In addition, as diamond is a good thermal conductor,it quickly draws heat away from the cutting tip, preventing diamond lossthrough thermal degradation (graphitization).

Once the wear progresses beyond this thick diamond edge, it passes intothe interior of the cutter. The diamond layer here is thinner and as thewear flat initiates, it will generate a sharp diamond lip, maintainingthe ability to shear the formation.

Another version of the cutter, again leached as described above, wouldbe a cutter with a plow shape (see, FIGS. 3A-3C which show variousviews, side, top and perspective, respectively). The plow shape wouldhave a sharper tip in the direction of bit rotation. This would aid inthe failure of the rock.

Another version of the cutter, again leached as described above, wouldbe an elliptical cutter with concave surface (see, FIGS. 4A-4D whichshow various views, two side, top and perspective, respectively). Theconcave surface is oriented through the large side of the cutter. Whencutter is engaged in the formation at a given DOC a sharper tip ispresented to the formation. With this type of shape the chip flow willbe different allowing better fluid flow and better cooling of thecutter.

Another version of the cutter, again leached as described above, wouldbe an elliptical cutter with concave surface (see, FIGS. 5A-4C whichshow various views, side, top and perspective, respectively) wherein theconcave surface is oriented through the small side of the cutter(compare to opposite orientation shown in FIGS. 4A-4D). The concavitycan be adjusted resulting in a different angle at the cutter tip. Withconventional cutters the front face is at 90° to the side of the cutter.With this type of shape we can break the relationship between back rakeand relief angle. For example in the above cutter, the angle at the tipis 15°. With a pocket back rake of 20°, we have a relief angle of 20°.But we have an actual back rake at the tip of 5°. Low back rakes areadvantageous in soft formation. Another example is with a relief andpocket angle of 10° resulting in a forward rake of 5°. Cutters of thistype are known to have a self penetrating effect. Difficulties in thatpast were obtaining a self penetrating cutting structure withoutweakening the supporting structure.

The following configurations are of importance to the presentinvention: 1) A cutter with a shape sharper than a conventionalcylindrical cutter with a leached layer on the working surfaces of thecutter; 2) A cutter with an elliptical shape with a leached layer on theworking surfaces of the cutter; 3) A cutter with a plow shape with aleached layer on the working surfaces of the cutter; 4) An LCA cutterwith a leached layer on the working surfaces of the cutter; 5) A cutterwith an elliptical shape and a concave front surface and with a leachedlayer on the working surfaces of the cutter; 6) A cutter with anelliptical shape and a convex front surface and with a leached layer onthe working surfaces of the cutter; 7) A cutter like any of the aboveformed on a cylindrical cutter with the working surface not parallel tothe back surface; and 8) Any of the above configurations with layers ofcoarse and fine diamond.

Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

1. A PDC cutter, comprising: a tungsten carbide substrate having a topsurface including a protrusion pattern formed in a center portion of thetop surface; and a diamond table mounted to the top surface of thetungsten carbide substrate; wherein the diamond table is thicker at aperimeter of the top surface and thinner at the center portion of thetop surface; and wherein the diamond table is leached.
 2. The cutter ofclaim 1 wherein the cutter has an elliptical shape having a major axisand a minor axis.
 3. The cutter of claim 2 wherein a top surface of thediamond table has a concave shape oriented through the major axis. 4.The cutter of claim 2 wherein a top surface of the diamond table has aconcave shape oriented through the minor axis.
 5. A PDC cutter,comprising: a tungsten carbide substrate; and a diamond table mounted tothe top surface of the tungsten carbide substrate, the diamond tablepresenting a chisel forward shape; wherein the cutter has an ellipticalshape ; and wherein the diamond table is leached.
 6. A PDC cutter,comprising: a tungsten carbide substrate having a top surface includinga protrusion pattern; and a diamond table mounted to the top surface ofthe tungsten carbide substrate; wherein the diamond table is thicker ata perimeter of the top surface and thinner at the center portion of thetop surface; and wherein the diamond table is leached.
 7. The cutter ofclaim 6 wherein the cutter has an elliptical shape having a major axisand a minor axis.
 8. The cutter of claim 7 wherein a top surface of thediamond table has a concave shape oriented through the major axis. 9.The cutter of claim 7 wherein a top surface of the diamond table has aconcave shape oriented through the minor axis.